1. Field of the Invention
This invention generally relates to a semiconductor laser, an optical element, a laser device, and a method of controlling the semiconductor laser.
2. Description of the Related Art
Many organizations have developed a wavelength changeable semiconductor laser having an element which can emit various wavelengths laser lights, as Wavelength Division Multiplexing (WDM) communication using an optical fiber has been spread. The wavelength changeable semiconductor lasers proposed before now are classified broadly into a semiconductor laser having a Semiconductor Optical Amplifier (SOA) in an external resonator thereof and controlling an emitting wavelength with a wavelength selecting mechanism in the external resonator and a semiconductor laser having a structure in which a resonator acting as a wavelength selectable portion is built in a semiconductor element amplifying a laser light.
A laser having a Sampled Grating Distributed Reflector (SG-DR) waveguide is disclosed as a typical wavelength changeable semiconductor laser, in which the resonator is built in the semiconductor element, in Japanese Patent Application Publication No. 9-270568 (hereinafter referred to as Document 1), Japanese Patent Application Publication No. 2004-336002 (hereinafter referred to as Document 2), Japanese Patent Application Publication No. 2003-17803 (hereinafter referred to as Document 3), U.S. Pat. No. 6,432,736 (hereinafter referred to as Document 4), U.S. Patent Application Publication No. 2003/0128724 (hereinafter referred to as Document 5), U.S. Patent Application Publication No. 2002/0105991 (hereinafter referred to as Document 6), U.S. Patent Application Publication No. 2002/0061047 (hereinafter referred to as Document 7), U.S. Pat. No. 6,590,924 (hereinafter referred to as Document 8), U.S. Pat. No. 6,317,539 (hereinafter referred to as Document 9), U.S. Pat. No. 4,896,325 (hereinafter referred to as Document 10), and Proc. IEEE, Selected Topics in Quantum Electronics, IEEE Journal of, Vol. 11, Issue 1, 2005 (hereinafter referred to as Document 11). The laser disclosed in Document 8 uses a vernier effect. The laser has a structure in which SG-DR waveguides having different optical longitudinal mode interval from each other are respectively connected to both ends of a waveguide amplifying a laser light. The laser changes a reflection peak wavelength of optical longitudinal modes of one of the SG-DR wavelengths and a reflection peak wavelength of optical longitudinal modes of the other by changing temperature, current and so on, and emits a laser at a wavelength where the peak wavelengths corresponds to each other.
The laser disclosed in Document 1 has a structure in which a SG-DR waveguide is combined to a Fabry-Perot (FP) resonator amplifying a laser light, changes reflection peak wavelengths of optical longitudinal mode of the FP resonator and the SG-DR waveguide by changing such as temperature or current, and emits a laser at a wavelength where the peak wavelengths correspond to each other.
Both of the lasers disclosed in Document 1 and Document 8 emit a laser based on a same principle using the vernier effect by the semiconductor waveguide. The laser disclosed in Document 1 has an advantage relative to that disclosed in Document 8, because an element length of the laser disclosed in Document 1 is smaller than that of the laser disclosed in Document 8. However, it is difficult that the laser in Document 1 emits a stable laser, because the Q value of reflection spectrum of the simple FP resonator is small. And a structure, in which a SG-DR waveguide is combined to a Sampled Grating Distributed Feedback (SG-DFB) laser having a SG-DR waveguide core as a gain medium has been proposed in Document 2.
The SG-DR waveguide has segments combined to each other, in which an area having a diffractive grating and an area not having a diffractive grating are combined to each other. The lengths of the segments in the conventional SG-DR wavelength are substantially equal to each other. When a wavelength is changed, the refraction index of each segment of the SG-DR waveguide is controlled in a same condition.
However, a conventional wavelength changeable laser having a SG-DR waveguide tends to emit lasers besides a desirable laser, when the range where wavelength is changeable is spread. There is, therefore, a problem that the mode stability of the laser emission is degraded. A description will be given of the reason with reference to reflection spectrums of two SG-DR waveguides having different optical longitudinal mode interval from each other.
It is necessary to match one of the optical longitudinal mode wavelengths of a SG-DR waveguide and one of the optical longitudinal mode wavelength of another SG-DR waveguide with a desirable wavelength or to match one of the optical longitudinal mode wavelengths of a SG-DR waveguide and one of the optical longitudinal mode wavelengths of a SG-DFB waveguide with the desirable wavelength. The optical longitudinal mode wavelengths of the SG-DR waveguide change when the refraction index of the SG-DR waveguide is changed. It is possible to change the refraction index by changing such as a current provided to the SG-DR waveguide or temperature of the SG-DR waveguide. For example, it is necessary to control the temperature of the SG-DR waveguide in a range of approximately 15 degrees C in order to match the optical longitudinal mode with a desirable wavelength with an element temperature, when the interval of the optical longitudinal modes of the SG-DR waveguide is approximately 200 GHz. A usual mechanism can achieve the temperature range.
FIG. 11 illustrates a calculation example of reflection spectrums of two SG-DR waveguides when the intervals of the optical longitudinal modes of the two SG-DR waveguides are respectively 194 GHz and 170 GHz. In this example, the temperatures of the two SG-DR waveguides are controlled so that the optical longitudinal modes of both SG-DR waveguides correspond at 194000 GHz. The horizontal axis of FIG. 11 indicates frequency. The vertical axis of FIG. 11 indicates a reflectivity of the SG-DR waveguides measured by dB. A laser emission is obtained at 194000 GHz, because both optical longitudinal modes of the SG-DR waveguides correspond to each other at 194000 GHz. However, the peak frequencies of both of the optical longitudinal modes discord to each other as the frequency gets away from 194000 GHz, because the intervals of the optical longitudinal modes are different from each other. No laser is emitted at other optical longitudinal mode frequencies around 194000 GHz. This is the vernier effect.
The peak reflections of the optical longitudinal modes of both of SG-DR waveguides correspond to each other approximately at 195400 GHz and 192600 GHz away from 194000 GHz by 1400 GHz. Lasers intend to be emitted at these two frequencies. Accordingly, the mode stability of a laser emission desired at 194000 GHz is degraded.
FIG. 12 illustrates an emission spectrum at threshold of laser emission calculated by a computer simulation for threshold by using the wavelength changeable semiconductor laser having two SG-DR waveguides shown in FIG. 11. The horizontal axis of FIG. 12 indicates frequency. The vertical axis of FIG. 12 indicates light intensity. As shown in FIG. 12, lasers are emitted at 195600 GHz.
It is necessary to reduce the difference between the optical longitudinal mode intervals of the two SG-DR waveguides so that the frequency where the reflection peaks correspond is out of the wavelength changeable range, and to structure a waveguide core having a gain getting smaller in a range out of the wavelength changeable range, in order to restrain the mode degradation caused by the mechanism. The wavelength changeable range is, for example, approximately from 192000 GHz through to 196000 GHz.
Next, a description will be given of a calculation example in a case where optical longitudinal mode intervals of the two SG-DR wavelengths are set respectively at 194 GHz and at 184 GHz and the difference between the intervals is 10 GHz. In this case, there is no correspondence between the reflection peaks in the range from 192000 GHz through to 196000 GHz besides at 194000 GHz. However, the difference between the optical longitudinal mode peak frequencies of the two SG-DR wavelengths is reduced at the optical longitudinal mode next to 194000 GHz. Accordingly, there is a problem that a laser tends to be emitted at the optical longitudinal mode next to the desirable frequency.
As mentioned above, there is a problem that an emission tends to occur at frequencies besides a desirable one and the mode stability of a laser emission is degraded in a conventional art, in a case where an element is structured so as to change a laser wavelength in a broad range.
A case where two SG-DR wavelengths are used is mentioned above. The same phenomenon occurs in a semiconductor laser in which a FP waveguide and a SG-DR waveguide are combined or a SG-DR waveguide and a SG-DFB waveguide are combined. As shown in FIG. 11, a whole point of the mode degradation in a conventional art is that the semiconductor has a little wavelength dependence of an optical longitudinal mode peak reflectivity of a reflection spectrum. That is, it is very difficult to restrain the mode degradation of a laser emission of a wavelength changeable laser having a SG-DR waveguide of the conventional art and being capable of changing a wavelength in a broad range.
An art to solve the problem, in which pitches of sampled diffractive gratings change from one end to the other end, is disclosed in Document 3. It is possible to enhance the reflectivity of an optical longitudinal mode around a given wavelength in the arrangement. However, it is necessary to use an advanced exposure technology using electron beam lithography in order to structure the diffractive gratings. In addition, it is very difficult to stabilize the phase relation between the sampled diffractive gratings of segments. There is a problem that interference effect of the reflected light from each segment is reduced.
The method of enhancing a reflectivity at a wavelength around a given wavelength by using a diffractive grating having various pitches is known, as mentioned in Document 11. However, the diffractive grating has no clear optical longitudinal modes like those of SG-DR waveguide. Therefore, there is a problem that it is difficult to restrain an emission of the FP waveguide, the SG-DR waveguide or the SG-DFB waveguide combined to the SG-DR wavelength at an optical longitudinal mode next to the desirable mode